1. Field of the Invention
The present invention relates to an x-ray image intensifier as well as the application of said intensifier to a digital radiology system.
2. Description of the Prior Art
X-ray image intensifiers are well-known in the prior art. By way of example, relevant information on this subject will be found in an article published in volume 8, No 4 of the December 1976 issue of the Thomson-CSF technical review, under the title "Image intensification in medical and industrial radiology".
The function of an x-ray image intensifier is to convert an x-ray image to an image which can be viewed on a screen. A typical apparatus of this type comprises:
a luminescent input screen for converting incident x-rays to light photons; PA1 a photocathode in optical contact with the luminescent screen for converting light photons to photoelectrons; PA1 an electron-optical system for focusing electron trajectories and producing a photoelectron energy gain; PA1 a viewing screen for the conversion of photoelectrons to light photons.
This invention is more particularly concerned with luminescent input screens for x-ray image intensifiers, hereinafter designated as X.I.I. tubes.
At the present time, these screens are usually formed by vacuum deposition, on a concave substrate, of luminescent material having a high atomic number such as cesium iodide.
In the majority of cases, known screens have either a greater thickness of luminescent material at the edges than at the center or a thickness which is substantially constant but rather greater at the edges than at the center.
FIG. 1 of the accompanying drawings is a cross-sectional view of a luminescent screen 1 having a thickness h.sub.b at the edges which is greater than the thickness h.sub.c at the center. The dashed-line curves a and b of FIG. 2 show that, in the case of known screens, the variation in thickness of the layer of luminescent material from the center to the edges expressed as a percentage of its thickness at the center of the screen is either increasing (curve a) or is substantially horizontal but exhibits a tendency to increase (curve b).